Summary: Psychophysical and physiological studies will be made on plasticity of human and animal visual systems. In humans who have considerable ocular astigmatism, a condition of meridional amblyopia can occur in which resolution for detail of specific orientations is reduced. Various visual functions will be investigated in meridional amblyopes in order to establish a functional basis of the defect. Physiological experiments are also planned. Cats will be reared under varying conditions of environment and visual deprivation. Single neurons in visual cortex will then be studied using basic neurophysiological methods. It is intended that the results will help to provide a physiological basis for some of the psychophysical findings.